This invention relates to an automatic iris adjustment system used for television cameras, video cameras, and the like.
Generally, the auto-iris circuit used in the television camera consists of a camera lens including an iris servo motor, an image pickup element placed inside the camera main unit, an image signal amplifier, a rectifying circuit, a voltage amplifier, and an iris servo amplifier. The circuit operates to control the iris value on a closed-loop basis so that the amount of light incident to the image pickup element is constant.
FIG. 1 shows in block diagram the conventional auto-iris control system. The light from a subject is incident to the camera lens 1, which has the iris value controlled by the servo motor 7. The light coming out of the lens 1 is incident to the image pickup element 2, and it is converted into a small-level electric signal. The signal is delivered to the image signal amplifier 3 and amplified to the prescribed level. The output of the image signal amplifier 3 is rectified by the rectifying circuit 4 so that it is converted into a d.c. voltage level. The produced d.c. voltage signal is compared with the reference voltage by the voltage amplifier 5, which then produces an amplified differential voltage. The output voltage signal is fed to the iris servo amplifier 6 so that it is subjected to power amplification. The servo motor 7 is driven by the output signal corresponding to the amplified differential voltage provided by the iris servo amplifier 6, and the iris value of the camera lens 1 is controlled such that the output d.c. voltage level of the rectifying circuit 4 is equal to the reference voltage.
This system forms a closed loop circuit, and the iris value is controlled so that the amount of light incident to the image pickup element 2 is constant. In the auto-iris circuit of this type, the response of iris value to the variation of incident light is adjusted by changing the amplification factor and frequency response of the voltage amplifier. The voltage amplifier is generally designed to have a large amplification factor so as to ensure the high response operation.
However, if the voltage amplification factor is too large, a swing-back phenomenon called "hunting" occurs when the amount of incident light has varied abruptly.
On this account, the voltage amplification factor and frequency response of the voltage amplifier are determined on a compromise basis in view of the response and hunting. In general, the occurrence of hunting is prevented by slightly sacrificing the response, which results in a noticeable delay of response appearing on the display.